Abstract: An active-hydrogen-containing (carboxyl or hydroxyl) phosphorus compound is provided. An epoxy resin semi-thermoset formed by bonding the phosphorus compound to an epoxy group is also provided. A flame-retardant epoxy resin thermoset is formed after reacting the epoxy resin semi-thermoset with a curing agent. The epoxy resin thermoset possesses excellent flame retardancy, heat stability, and high glass transition temperature (Tg), does not produce toxic and corrosive fumes during combustion, and thus is an environmentally friendly flame-retardant material.